Drilling wellbores in the earth, such as wellbores used for the production of oil and gas, is a well established art. One type of drilling system is rotary drilling, which uses a drill bit at the end of a drill string to drill into the earth. At the surface, a drilling rig controls the position and rotation of the drill string below the surface. Underneath the surface, the drill bit is attached to the drill string that transports drilling fluid to the drill bit. The drilling fluid lubricates and cools the drill bit and also functions to remove cuttings and debris from the wellbore as it is being drilled.
While simple rotary drilling has been employed for many years, directional drilling is becoming a more common drilling practice. Directional drilling involves changing the direction of drilling as needed to reach a desired wellbore endpoint, or to create a desired wellbore pattern. For example, a whipstock may be inserted into the wellbore and used to deflect the drill bit in the desired direction. Another type of directional drilling involves the use of bent motors in which a slight curvature of the bent motor allows steering of the direction of the wellbore. To steer using a bent motor, rotation of the drill string is halted while allowing the drill bit to continue to rotate. Because the bent motor is slightly angled and because the drill string is not rotating, the drill string is effectively steered in the direction of the bend of the motor as the drill bit continues to move forward. This “directional drilling” may be difficult due to static friction between the non-rotating drill string and wall of the wellbore, especially for long drill strings.
Prior techniques for overcoming this static friction condition include “rocking” or “winding up” the drill string. This process utilizes the torsional flexibility of the drill pipe to allow short, cyclical reversing of the direction of rotation of the drill pipe. In this process, the drill pipe is quickly rotated back-and-forth at the surface, yet borehole friction prevents the torque from being transmitted to, or changing the orientation of the bent motor assembly. Vibrating the pipe with either a surface or down-hole vibrating device may also be employed to overcome static friction. Additionally, rotary steerable systems may be used, in which the entire drill string continues to rotate while adjustable near-bit stabilizers force the drill pipe to become eccentric within the wellbore, thus causing wellbore deviation to take place.